1. Field of the Invention
The invention relates to a method for determining at least one characteristic for the correction of measurements of a Coriolis mass flowmeter. The invention further relates to a Coriolis mass flowmeter having at least one measuring tube, at least two sensors and at least one actuator, wherein the measuring tube can be excited to at least one oscillation with the actuator.
2. Description of Related Art
Coriolis mass flowmeters of the type above have been known for a long time and, due to their measuring principle are theoretically unsusceptible to the properties of the measuring medium, such as e.g., thermal conductivity, thermal capacity and viscosity. They measure the mass flow directly by evaluating a mechanical oscillation of the measuring tube influenced by the mass flow.
However, in commercial use, especially for Coriolis mass flowmeters having a single measuring tube, the problem often arises that there is a change in zero point and sensitivity during operation. This change can, in particular, be ascribed to the coupling of the oscillation system with its environment. A change in the environmental conditions leads to a change in the characteristic properties of the measuring system. For this reason, both the zero point and the sensitivity have to be corrected in consideration of the current operating and mounting conditions in order to be able to ensure a required high accuracy.
The central element of Coriolis mass flowmeters is the measuring tube, which has medium flowing through it of which the mass is recorded. Coriolis mass flowmeters having multiple measuring tubes—straight or curved—are also known. For the sake of simplicity, one single measuring tube is discussed in the following, however, the implementations are also valid without limitations for Coriolis mass flowmeters having multiple measuring tubes. The measuring tube is rigidly anchored at its ends and is excited to oscillation in resonance in its first eigenmode by an electro-magnetic actuator. The detection of the measuring tube oscillation often occurs using two electro-magnetic sensors. In the ideal case, both measuring tube halves oscillate synchronously without mass flow. As soon as mass flow occurs through the measuring tube, Coriolis forces occur due to the velocities of the movement of the measuring tube, on the one hand, and the movement of the mass particles in the measuring tube, on the other hand, being orthogonal to one another. These are oppositely oriented relative to one another on the in-flowing and out-flowing side of the measuring tube, so that the oscillation of the measuring tube half is advanced on the in-flowing side in respect to the movement of the oscillation of the measuring tube half on the out-flowing side.
The phase-shift of the oscillation of the measuring tube occurring during mass flow, which is ultimately seen in a time delay of the oscillation between the two measuring tube halves, is proportional to the mass flow and is, thus, used in determining the measurements. In particular, for this reason, it is necessary to have precise knowledge of the oscillation properties of the measuring system, especially the first eigenmode at zero flow. In the sense mentioned above, the phase difference between the oscillation of the measuring tube halves—or other sections of the measuring tube, which can depend on the eigenmode, in which the measuring tube is excited—is a measurand, even if it is a measurand derived from the detected oscillation of the measuring tube. Coriolis mass flowmeters often show a phase difference other than zero, even at zero flow, and consequently, deviate from theoretical behavior. In order to ensure a certain degree of accuracy, this knowledge of the zero point offset of the phase is of particular interest in practice.
In the prior art, the time difference resulting from the phase difference of the oscillation is determined between the two sensors and a mean is calculated from the determined values, which is used for correcting the measurements. The methods for a determination of characteristics for correcting the measurements of a Coriolis mass flowmeter known from the prior art are, however, prone to errors and often less exact, which is why they lead to inaccuracies even with the already corrected measurements of the Coriolis mass flowmeter.